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Point and shoot: a brief look at crop sensors for N management

Knowing in-crop nitrogen (N) status is a critical part of good crop nutrition. Crop N sensors can give a real-time estimate of plant N status. This helps gauge if N levels are excessive or deficient (ideally before yield loss occurs). 

How sensors work

Sensors don’t measure N content directly. They infer N from how ‘green’ the crop is by measuring the light reflected off the crop canopy. Healthy or vigorous crops absorb lots of red and blue light, and reflect lots of near-infrared (NIR) light. Poorer crops reflect more red and blue light. Sensors convert these light readings into an index, such as the normalised difference vegetation index (NDVI). 

NDVI values and crop N status are correlated. Very green crops have higher NDVI values, and an assumed higher N content. 

What affects readings?

Stresses other than low N can affect readings. Water stress, disease, and other nutritional deficiencies can influence the results. Readings are also influenced by crop variety, row spacing, weeds, and waterlogging. Moisture on the leaf reduces reflectance, giving lower NDVI values. 

Using crop sensors

One SA grower is trialling sensors with N-rich and 0-N strips. Sensors scan the paddock during the season to check if N is becoming limiting. Comparing readings on and off the strips helps inform N recommendations. The N-rich strip also shows if the difference in NDVI readings is caused by variable N uptake or another limitation such as other nutrient deficiencies.

Most of the available commercial sensors in Australia can be tractor or boom mounted. Sensors need to be mounted to maximise the amount of light received. Position sensors directly over the crop row, so as much of the crop as possible is contributing to the readings.  

Crop sensors and yield monitoring

Crop sensor data such as NDVI can be compared to a number of other paddock level data sets. This may help diagnose the reasons for NDVI variability and yield outcomes. Common comparisons include:

  • apparent electrical conductivity
  • yield
  • variable rate N applications
  • soil data e.g pH maps.

As an example, low yield and high NDVI could indicate weed pressure. Or it could be from a large biomass crop running out of moisture and unsuccessful grainfill. Each situation will be different, requiring several layers of data and ground-truthing to unpack what’s happening in the paddock. 

While sensors are improving rapidly, they’re not perfect yet. The most accurate way to measure plant or soil N is sampling and laboratory analysis. Sensors are a tool to help crop nutrition decisions, and should not be used in isolation. If you use sensors, calibrate them to your site, the crop, and the season. 


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Proximal crop reflectance sensors

Watch: Future Farm

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